Researchers from the Centre of Fish and Fisheries in Perth, Australia, have discovered that goldfish the 'size of footballs' are travelling hundreds of kilometres each year in Western Australian waterways. The fish, which can weigh up to 1.9kg in total, are having a significant impact on the ecosystems of their new habitats, they say.

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Goldfish are native to eastern Asia but are regularly dumped from aquariums into catchment lakes. Once the fish become established, their eradication is difficult. They can impact water quality, introduce disease, disturb habitats and compete with other fish with can cause breeding problems for the native species.

“Once established, self-sustaining populations of alien freshwater fishes often thrive and can spread into new regions, which is having a fundamental ecological impact and are major drivers of the decline of aquatic fauna,” said Dr Stephen Beatty, author of the study, in a statement.

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The domesticated fish are considered one of the world’s worst invasive aquatic species, with at least 76 invasive freshwater fish species introduced to rivers in Mediterranean climate regions around the world.

Dr Stephen Beatty with one of the extremely large goldfish

Murdoch University

The study, published in Ecology of Freshwater Fish, outlines how the researchers used strategically placed acoustic receivers to examine the movement patterns of introduced goldfish in the Vasse river. The team discovered the goldfish fish can swim up to 230km every year. In particular, one problem with goldfish is that they are carnivorous, and can eat the eggs of other fish, which can cause population decline.

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“The goldfish population in the nutrient rich Vasse River has existed for over two decades and has the fastest known individual growth rate of this species in the world,” Beatty said..

“The results of this study will have important direct management implications, enabling more strategic development of effective control programs for the species such as targeting migratory pathways.”

Invasive goldfish have also been a problem in California, Canada and Colorado. Canadian authorities in Alberta made a plea to the public to not dump their unwanted fish in ponds because of the impact it was having on the ecosystem.